1. Technical Field
The present invention relates generally to paving and, more particularly, to a paving machine and pavement edger therefor.
2. Related Art
Typically, screed pavers include a self-propelled paving machine having a hopper for receiving paving material, e.g., asphalt, from a truck so that the truck progressively dumps its load of paving material into the hopper. A conveyor system on the paving machine transfers paving material from the hopper rearwardly for discharge onto the roadbed in front of transversely arranged screw augers which spread the material laterally in front of a main screed. This main screed functions to compress and level the paving material distributed by the augers to give a smooth finished road surface. The height and attack angle of the main screed may be varied to control the depth and surface of the pavement mat. The main screed may also include screed extenders to allow for a wider pavement mat to be laid.
One of the problems in paving of multiple lanes, especially on high speed interstate highways, is the drop off at an edge of a new pavement mat. During paving operations, it is oftentimes impossible to pave two lanes in a short time span due to a variety of reasons, e.g., traffic, equipment shortages, etc. One reason, in particular, is time constraints caused by the paving crew having to back up and start the second or closure pass on a two lane paving operation at mid-day. Where more than two lanes are being paved, the paving crew must back up at least twice during the day to minimize drop off length on both lanes being paved. Despite the drop off problem, it has become common practice for paving crews to pave only a single lane during one paving day to avoid having to back up. The entire length of this pass therefore becomes a drop off. Where an edge must be left overnight, a drop off of up to 11/2 inches has not been considered objectionable for a short distance.
While a drop-off is usually only an overnight or weekend problem, it creates safety problems such as: vehicle wheels becoming caught on the drop off during lane changes onto or from the new mat, and loose stones/aggregate being kicked up by vehicles. In response to these safety problems, federal and some state highway contracting regulations are now mandating that any drop off between a new pavement mat and any adjacent material, e.g., un-repaved asphalt, shall not have a height over one inch unless a paved ramp is provided from/to the new pavement mat. Because it is often highly undesirable to lay a new layer of pavement of an inch or less, in most cases when one lane is laid, it must be provided with a ramp.
Ramps, unfortunately, create a number of other problems. One problem is at the beginning or ending of a mat, the wedge section must be adjusted manually during the transition, thus increasing the potential for an unacceptable section of pavement. Another problem with ramps is that they make it more difficult to create solid joints.
To address the joint creation and drop off problems, the concept of the "tapered joint" ramp was developed. At least two versions of tapered joints are in use: First, as shown in FIG. 1A, the "Jersey Unit," as developed in the state of New Jersey during the 1980's, includes a first pavement mat 6 including a ramp 1 having a tapered portion 2 extending from a surface 3 of an adjacent and/or underlying material 4 directly up to a horizontal surface 5 of the pavement mat 6. Second, as shown in FIG. 1B, the "Stepped Tapered Joint," as currently used in the state of Michigan, includes a first pavement mat 16 including a ramp 11 having a tapered portion 12 extending from a step 12B to a second step 12A on a surface 13 of an adjacent and/or underlying material 14. The stepped tapered joint is basically a stepped jersey unit.
While tapered joint ramps cure the drop off problem, it unfortunately remains extremely difficult to form a solid long-lasting joint for the reasons that follow.
In terms of the jersey unit, a number of problems arise:
First, traffic which crosses over tapered portion 2 of ramp 1 partially compacts a line 7 between horizontal portion 5 of pavement mat 6 and tapered portion 2 of ramp 1. This compaction makes it very difficult or impossible to discern the actual edge of mat 6 during laying of a second pavement mat 8, shown in phantom in FIG. 1A. As a result, either ramp 1 must be removed or very precise paving machine operation is required to follow an almost non-existent edge 7 of first pavement mat 6. When second pavement mat 8 is laid over ramp 1, frequently the result is a feathered joint 9 where second pavement mat 8 lays over ramp 1 but does not have its edge meet cleanly with edge 7 of first pavement mat 6, i.e., either second pavement mat 8 is short of edge 7 or passes over edge 7. Feathered joint 9 is problematic because it may include a visible rut between pavement mats that can lead to deterioration and ravel under traffic. Additionally, water may gain easy access through feathered joint 9 and under second pavement mat 8 which may cause roadway heaving or separation problems.
Second, full compaction is oftentimes only applied to the horizontal part of first pavement mat 6. Tapered portion 2 of ramp 1 is normally only exposed to that compaction provided by the screed that forms it and whatever traffic crosses it. See e.g., U.S. Pat. No. 4,181,449 to Lenker, and U.S. Pat. No. 4,818,140 to Carlson. As a result, tapered portion 2 includes a low density area 10 which by the time second pavement mat 8 is laid has cooled and is extremely resistant to further compaction. Second pavement mat 8 does not contain a sufficient amount of hot material over low density area 10 to allow further compaction. The resulting joint therefore is immediately suspect.
Third, because the outermost extent of tapered portion 2 must be created by pavement material at its core particle size, e.g., small stones, it is oftentimes impossible to construct the outermost extent of tapered portion 2 such that it irremovably compacts into the rest of tapered portion 2 and/or adjacent/underlayer material 4. As a result, a loose aggregate safety problem persists.
Referring to FIG. 1B, the stepped tapered joint ramp was developed to alleviate the problems of raveling and edge following. By providing a step 12B at an edge 17 of new pavement mat 16, a feathered edge is prevented. Further, step 12B provides a defined line or edge 17 which alleviates the problem of having to follow an undecipherable compacted edge of first pavement mat 16. Unfortunately, the compaction problem for the tapered or wedge section 12 remains, i.e., a low density area 20 that is resistant to compaction exists. Further, if the proper height for step 12B is not incorporated, e.g., because different asphalt formulations have different compaction ratios, step 12B can be rolled out of existence when the rest of first pavement mat 16 is compacted.
One remedy for the joint creation problems of ramps has been to remove the ramps prior to laying the second pavement mat. Unfortunately, this process is very time consuming and difficult because the material has cooled and hardened. It may also necessitate additional lane closure to accommodate equipment.
In view of the foregoing, there is a need for a paving machine and pavement edger therefor which allow for accommodation of drop off from a new pavement mat and the creation of solid pavement joints.